For example, a typical automobile wheel made of steel includes: a substantially disk-shaped wheel disk; and a substantially cylindrical wheel rim. The wheel disk includes: a hub mounting portion that is connected to a hub of an axle; a hat portion that bulges from an outer circumferential edge of the hub mounting portion toward a surface side direction; and a disk flange that extends from an outer circumferential edge of the hat portion toward a back side direction. The hub mounting portion, the hat portion, and the disk flange are arranged concentrically. The wheel disk is engaged with an inner circumferential surface of the wheel rim to be welded to the wheel rim together into one piece. The hat portion of the wheel disk offers desired stiffness. The hat portion of the desired stiffness is very effective in minimizing deformation of the wheel disk due to various loads that are imposed during car driving. Thus, the wheel disk offers high overall strength. Particularly, under lateral force loading on the car during driving, a bending moment acts on the wheel disk. The bending moment is relatively large enough to bend the wheel disk in a surface side to back side direction. The hat portion offers high stiffness for the bending moment and minimizes the deformation of the wheel disk. This allows the automobile wheel to offer high strength and to provide increased driving stability.
Such automobile wheel is proposed in, for example, as described in Japanese Unexamined Patent Publication No. 2001-180202 (“JP '202”). In JP '202, the hat portion includes: an inward angled circumferential portion; an outward angled circumferential portion; and a hat top circumferential portion. The inward angled circumferential portion includes: an inward protruding circumferential portion that is curved toward the surface side; and an inward recessed circumferential portion that is curved toward the back side. The outward angled circumferential portion includes an outward recessed circumferential portion that is curved toward the back side. The hat top circumferential portion protrudes toward a surface side direction between the outward angled circumferential portion and the inward angled circumferential portion. In the automobile wheel of the aforementioned construction, the hat portion is formed into the shape described to allow the automobile wheel to have high stiffness, and thus to offer high overall strength.